The subject of this invention is an ion beam preparation device for processing electron microscopy specimens with a vacuum chamber and at least one ion source for specimen bombardment in a specimen holder by a noble gas ion beam, in particular with argon ions.
For electronmicroscopical observation of specimens the latter must be suitably prepared, e.g. by ion beam etching (D. G. Howit,, Ion Milling of Materials Science Specimens for Electron Microscopy: A Review, Journal of Electron Microscopy Technique 1: 405-414 (1984); A. Garulli, A. Armigliato, M. Vanzi, Preparation of Silicon Specimens for Transmission Electron Microscopy, J. Microsc. Spectrosc. Electron. Vol. 10, No. 2, 1985, 135-144).
Ion etching for preparing specimens used in scanning electron microscopy (SEM) and transmission electron microscopy (TEM) is a method that is principally used where conventional chemical and electrochemical processes fail or yield only inadequate preparation results. This applies in particular to TEM cross-sectional preparation of material and layer combinations with strongly selective etching behavior, and to chemically resistant materials. For these cases ion etching has developed into a routinely practiced method.
Whereas in the beginning cross-section specimens were usually examined with transmission electron microscopes with an acceleration voltage of 100 kV, preference is now given to medium voltage equipment with 300 kV and field emission sources. This equipment ensures uniform transmissibility of the cross-section specimens and is able to form beam probes in the nm range. This establishes the technical preconditions for structural examination and material analysis (EELS, EDX) in the finest details, that is, also on nanostructures.
The development of the nanotechnology, that is, the creation and utilization of structures and dimensions in the submicrometer and nanometer range (e.g. semiconductor component structures), imposes significantly more demanding requirements on the preparation technique. The necessary etching to the desired thickness of structures with extremely small dimensions requires a significantly better observation possibility of the specimen during the etching process so that the momentary stadium of the specimen preparation can be accurately determined.
The currently known, conventional ion beam etching systems such as the RES 010 from BAL-TEC, the PIPS model 691 and the Dual Ion Mill from Gatan, as well as the Ion Beam Thinning Unit from Technology LINDA, use light microscopy with a maximum magnification factor of 100 for observing the specimens. This is generally inadequate already in the cross-section preparation of simple multilayer systems because the moment at which the etching process is terminated cannot be accurately determined. In the case of specimen etching to the desired thickness of selected structures, in-situ evaluation of whether or not the structure of interest is located within the thinned down specimen area is entirely impossible. This applies already to structures (also samples with periodic structures) in the .mu.m range! As a consequence an elaborate "trial and error" process is needed in which the specimen must be repeatedly transferred between the etching system and the transmission electron microscope or the corresponding specimen holders. This often results in destruction of the specimen.
Another disadvantage of the known ion beam etching systems is the poor controllability of the final stage in the etching process. Neither the optical observability nor the automatic cut-outs known from the known ion beam etching systems allow determination of the exact etching process termination. The sensitivity of the optical and electronic cut-offs used in these equipments is inadequate for switching off the etching process on time. This applies in particular to the ion beam preparation of cross-section specimens. A certain improvement is achieved by the RES 010 from BAL-TEC which uses a special specimen holder with built-in Faraday cup for detecting all charged particles. However, this arrangement severely restricts the possibilities of the specimen holder because the specimen cannot be thinned and observed on the back.
In the known ion beam preparation devices the specimens are usually rotated during the etching process in order to improve the uniformity of erosion. From patent application U.S. Pat, No. 4,128,765 it is known that the specimens should not only be rotated but also the incidence angle of the ion beam should be varied during the etching process based on a random function. This is achieved by a rigid arrangement of the ion sources and by reciprocating the specimen holder containing the probe relative to the ion beam by a certain angle. After the etching process the specimen is ready for electronmicroscopical observation.
In addition to the conventional etching technique described above there is another technique that was initially developed for fault analysis on microelectronic circuits. With the aid of a finely focussed (diam. in nm range) scanning ion beam the specimen can be etched to the desired thickness and observed also through ion microscopy. This focussed ion beam technique (FIB) is currently used also for the preparation of specimens for scanning electron microscopy (SEM) and transmission electron microscopy. For this purpose the specimen areas of interest are partially cut through etching by means of Ga liquid metal ion sources with extremely high ion densities of up to 10 A/cm.sup.2, either on one side (slope for SEM examination) or on both sides (ribs for TEM examination). In-situ observation of the etching process is performed with the secondary particles detached by the ion beam (e.g. Hitachi FB 2000, FEI FIB 200, 600 and 800) and lately also with an additional scanning electron microscope (FEI Dual Beam FIB/SEM workstation). The finely focused ion beam and the ability of accurately positioning the specimen and the ion beam allow accurate etching of the specimen to the desired thickness. However, as ion beam etching is only possible with a stationary specimen this results in strongly preferred structures on the etching slope. This is particularly disadvantageous in multilayer systems with strongly selective etching characteristics. The necessarily high ion current density results in strong back-coating of the etching slope and strong heating of the specimen. TEM specimens can only be produced as ribs with a thickness of approx. 100 nm which renders high-resolution TEM (HRTEM)examination in selected specimen areas impossible. As the ribs must be held in place by the remaining specimen material, tilting of the specimen in the TEM for exact specimen orientation is possible only within narrow limits due to the shading effect. For producing TEM specimens the specimen surface must be coated. This FIB (focused ion beam) technique has become known, for example, from the Japanese patent application JP 6231720 which corresponds to U.S. Pat. No. 5,525,806. A strongly focused ion beam is scanned at an angle of 90.degree. across the substrate to be processed, where cuboid areas are worked out of the substrate, leaving thin ribs that form the area of interest for subsequent TEM examination. The arrangement with the scanning ion beam is operated simultaneously as a SIM, that is, as an ion microscope, for observing the etching process. After sufficient etching has occurred on both sides of the rib-shaped area of interest, the area of interest is exposed and the ion source can be switched off. The specimen can subsequently be examined with the SEM.
Simultaneous observation of the specimen during the etching process is possible only with the Dual Beam FEI in SEM mode. In the case of all other equipment the specimen must be moved into an observation position and can be observed only with the ion microscope. Even if a lower ion current density and acceleration voltage are used, the disadvantage is that during the observation material is removed from the specimen area of interest or that bombardment ions are implanted, both of which alter the original specimen material.